This invention relates to an engine air supply system for an aircraft. More particularly, the invention relates to a check valve for use in a gas turbine engine bleed system, for example.
Modern high performance fighter aircraft engines require a fast acting pneumatic check valve to prevent engine core bleed flow from returning to the engine during a compressor stall. Under one typical normal operating condition, bleed flow is extracted from a high pressure compressor stage and is delivered to the airframe for various environmental control system purposes, for example. The bleed flow from the engine can operate at over 40 atmospheres and at over 1100° F. with flows over 1 pps. Compressor stall recovery is adversely affected by any returning bleed flow from the environmental control system, and thus, a fast acting check valve improves surge recovery. Closure times of under 100 ms are typically desired.
Check valves that operate in this type of environment are typically semi-circular centrally hinged discs. The bleed discharge flow maintains the discs in an open position with an included angle of typically around 20 degrees. During a compressor stall when the flow reverses, the discs will move approximately 70 degrees from the open position to the closed position and strike the valve seat. The discs are coplanar in the closed position and perpendicular to the flow direction. The angular velocity at impact may reach many thousands of radians per second as the discs accelerate to the velocity of the air flowing in the reverse direction. Because the velocity at impact is directly related to the backflow air velocity, any reduction in angular travel for the discs to close will reduce the angular velocity on contact with the valve seat. This will both reduce the stress levels on the discs and seat and also reduce the amount of backflow.
One typical approach to addressing the above problem is to reduce the angular velocity by reducing disc travel. This is accomplished by having the discs less open in the open position than the arrangement described above. The included angle is reduced to around 30-40 degrees when in the open position to reduce the angular travel for the discs. The disadvantage of this approach is that under normal operating conditions of forward flow, the pressure drop across the valve will increase. Less pressure is then available for the aircraft systems, which may require larger components to accept the lower pressure or limit operating conditions.
What is needed is a check valve that reduces the stress levels between the discs and seat without increasing the restriction provided by the check valve.